1. Field of the Invention
The present invention relates to lubricating compositions. In particular, the present invention relates to a lubricant composition obtained by compounding molybdenum dithiocarbamate, molybdenum dithiophosphate, and/or a molybdenum amine compound; and a (poly)glycerol ether and/or a (poly)oxyalkylene glycol monoalkyl ether, in a base oil. More particularly, the present invention relates to a lubricating oil composition which exhibits excellent stability to hydrolysis and excellent friction reduction even after deterioration with water, and a grease which is used for universal joints including constant velocity joints (CVJ) for automobiles, constant velocity gears, and transmission gears, and which has excellent friction and abrasion properties.
2. Description of the Related Art
The automotive field is today confronted with strict fuel regulations, and exhaust gas regulations, etc. against the background of environmental pollution, e.g. global greenhouse effect, air pollution, and acid rain, and in order to preserve limited petroleum resources from exhaustive use. Improvements in mileage are the most effective way to respond to such regulations at present.
Improvements in engine oil, such as low viscosity engine oils and the addition of friction modifiers, as well as improvements in automobiles themselves, e.g. light weight vehicles and improved engines, are important means for achieving low fuel consumption in the automotive field. Engine oil acts as a lubricant between pistons and liners, and friction loss can be reduced by decreasing the viscosity of the engine oil due to the high fluid lubrication in this portion. However, the decreases in oil viscosity in recent years have also created such problems as deteriorated sealing properties and accelerated wear. Engine oil also plays an important role in the valve train and bearings. Low viscosity oil will cause increased wear due to mixed lubrication or boundary lubrication in these systems. Friction modifiers and extreme pressure agents are added to the oil to decrease friction and prevent wear.
Generally used friction modifiers include, for example, higher fatty acids, e.g. oleic acid and stearic acid; higher alcohols, e.g. oleyl alcohol; esters; amines; sulfide oils; chlorinated oils; and organic molybdenum compounds. Generally used extreme pressure agents include, for example, sulfide oils; sulfur compounds, e.g. sulfides; phosphorous compounds; and organic metals e.g. zinc dithiophosphate (ZnDTP).
For example, Japanese Laid-Open Patent No. 59-25890 discloses glycerin monoalkyl ether or glycerin monoalkenyl ether as the friction modifier, as well as a common lubricant composition produced by combining ZnDTP with an ash-free detergent-dispersant.
The addition of organic molybdenum friction modifiers providing low friction in mixed or boundary lubrication is inevitable in order to solve all the problems associated with the lowering of lubricating oil viscosity. Japanese Laid-Open Patent No. 5-279686 proposes an improvement in frictional properties without deterioration in other properties such as abrasion resistance by compounding an organic molybdenum compound; a fatty acid ester; a metallic detergent, such as calcium sulfonate, magnesium sulfonate, calcium phenate, and magnesium phenate; an ash-free detergent-dispersant, such as benzylamine and its boron derivative, and alkenylsuccinic imide and its boron derivative; and wear improvers such as ZnDTP and zinc dithiocarbamate (ZnDTC).
Alternatively, Japanese Laid-Open Patent No. 5-311186 discloses a drastic decrease in the friction coefficient of lubricating oil which contains a combination of a metal dithiocarbamate and an oil-soluble amine; sulfoxy molybdenum dithiocarbamate and/or sulfoxy molybdenum organophosphorodithioate; and a fatty acid ester and/or organic amides, in a selected ratio.
However, neither of the compositions disclosed in Japanese Laid-Open Patent Nos. 5-279686 and 5-311186 show reduced friction when oil contains water even with the addition of the molybdenum compound.
Inclusion of water in an engine oil formed during fuel combustion is inevitable. In particular, when engine oil is not heated, that is during repeated short distance operation cycles water content in the engine oil increases as the water does not evaporate. Water causes not only deterioration of the additives but also the activation of blow-by gas, resulting in significantly adverse effects on the engine oil. Thus, the development of an oil which can maintain decreased friction while maintaining fuel saving performance with little deterioration even when water is included has been needed.
Recently, CVJs have been widely employed for front engine front drive vehicles, four wheel drive vehicles, and front engine rear drive vehicles with independent suspension. CVJs are used to transmit power from the engine to the wheels, and the power must be smoothly transmitted even during steering. Thus, a CVJ generally consists of a combination of a plunging-type joint at the engine side capable of sliding in the axial direction and a fixed-type joint fixed in the axial direction at the wheel side. Since the sliding friction in the rotational direction occurs through the rolling-sliding motion during the reciprocating motion in the plunging-type joint, various noises and vibrations, e.g. vibrations during idling in an automatic transmission vehicle, lateral vibration during starting and accelerating, beat oscillations at certain speeds, and booming occur. Decreased vibration is an important issue to be solved for the development of more comfortable and quieter vehicles. Thus, not only has the joint itself been improved to decrease such vibrations, but the grease filled in the joint as well.
As there is a correlation between the vibration and the friction coefficient, and further as reduced fuel consumption is increasingly demanded, greases for providing decreased friction are being sought.
Molybdenum disulfide, sulfur-phosphorous additives, and lead additives have been conventionally used in grease for CVJs. Recently, organic molybdenum compounds have been used instead of the above additives, in order to produce grease exhibiting lower vibration or lower friction. Japanese Laid-Open Patent No. 6-184583 discloses a grease composition for CVJs comprising a urea grease, molybdenum dithiophosphate, molybdenum dithiocarbamate, and ZnDTC. Additionally, Japanese Laid-Open Patent No. 4-178499 discloses a grease composition for CVJs comprising a urea thickener, sulfurized molybdenum dialkyldithiocabamate, zinc dithiophosphate, and sorbitan fatty acid esters.
Although, long drain lubricating oils are now desirable with the aim of achieving a maintanance free lubricating composition, it is becoming an important problem to maintain this in addition to reduced fuel consumption. Engine oils undergo the most severe oxidative deterioration among lubricating oils, and the deterioration starts with the running of the vehicles. Additives also deteriorate along with this oil deterioration. Thus, improvements in the additives are also essential for maintaining the fuel saving properties of lubricating oil. That is, because the use of oil-soluble molybdenum compounds is essential for fuel savings, it is even more important to effectively draw out and maintain the properties of the molybdenum compounds.
Further, the friction of the grease compositions set forth above is not satisfactory and must be further lowered. Demand on greases has shifted to increasingly severe site conditions due to the decreased quantity of grease fillable in smaller and light weight CVJs, higher power output and higher vehicle speeds. Thus, low frictional performance is required for such greases in addition to high durability and high friction resistance.